1. Field of the Invention
The present invention relates to a method for discriminating between different types of optical discs, more particularly to discriminating between different types of optical discs in accordance with the relationships created after the synthesis of push-pull signals by the pick-up head (PUH) of an optical storage device.
2. Description of the Related Art
The great progress in information technology has led to a ceaseless increase in the number of ways to store data. In this regard, the optical storage device has apparently become an important data storage tool available on the market. For an optical storage device to operate, an electromechanical actuator drives a pick-up head so as to focus laser beams on an optical disk. Then, any saved binary data is read and judged by the magnitude of the light rays reflected to a photo-detector; meanwhile, the reflected rays function as servo control signals, for example, tracking error signals and focus error signals, for driving the pick-up head. In other words, the laser spots are accurately focused on a right track, using such an optical signal to drive the pick-up head.
FIG. 1 is a schematic diagram of the optical system of the pick-up head in an optical storage device. A laser ray 16 is generated by a laser diode (LD) 11 and then, with a phase grating 12, divided into three rays of light, namely a leading sub-beam 16b, a main beam 16a and a lagging sub-beam 16c. Then the three rays of light pass through a beam splitter 13 and an objective lens 14 before they fall on an optical disc 80 and form a secondary light spot 162, a primary light spot 161 and a secondary light spot 163 in a groove 81 and lands 82 respectively.
Three reflective rays are formed where each of the three light spots appears. Then the reflective rays are directed to a photo-detector 15 by the beam splitter 13. Data of the reflective rays are received by a primary receipt portion 151 and secondary receipt portions 152 and 153 of the photo-detector 15, respectively. As shown in FIG. 1, each of the receipt portions is further divided into two regions, wherein the primary receipt portion 151 comprises region A and region B, the secondary receipt portion 152 comprises region C and region D, and the secondary receipt portion 153 comprises region E and region F. Hence, it is feasible to acquire synthesized signals that conform to the following equations:MPP=A−B; SPP=(C+E)−(D+F);                where MPP denotes a main beam push-pull signal, SPP denotes a sub-beam push-pull signal, and A through F denote optical signals read by respective optical receipt regions.        
Conventionally, an optical storage device determines whether or not an optical disc is a DVD-RAM (Digital Versatile Disc-Random Access Memory) disc, using the Differential Push-Pull (DPP) method, that is, subtracting a sub-beam push-pull signal from a main beam push-pull signal in order to obtain a differential push-pull signal, as expressed with the following equation:DPP=MPP−SPP=(A−B)−{(C+E)−(D+F)}
If the peak-to-peak voltage of the DPP signal is greater than a default threshold value, it is judged that the optical disc being read is a DVD-RAM disc, otherwise the optical disc is categorized as another type of DVD, such as DVD-ROM (Read-Only Memory), DVD-R (Recordable), or DVD-RW (Rewritable) disc. However, a variety of DVDs take considerable discrepancies between conditions for light ray reflection. Thus, with only one threshold value setting, it is rather difficult to discriminate between different types of DVDs, and it may even be unlikely that every attempt to discriminate between different types of DVDs will bring an inaccurate judgment.